Noise reduction system



Feb. 14, 1961 M. E. HAWLEY mm. 2,972,

NOISE REDUCTION SYSTEM Filed Nov. 30. 1953 P/VASE Jll/FT 4170477151047! lift/MA ,7

INVENTORS ATTORNEY United States PatentO NOISE REDUCTION SYSTEM Mones E. Hawley, Collingswood, and Elvin D. Simshauser, Merchantville, NJ, assignors to Radio Corporation of America, a corporation of Delaware Filed Nov. 30, 1953, Ser. No. 394,918

8 Claims. (Cl. 179-1) This invention relates to acoustic devices, and more particularly to an improved means for reducing acoustic noise reaching a person wearing earphones.

- In many uses and applications of earphones used in amplifying systems, it is desirable to decrease the amount of extraneous noise reaching the ears of a listener. This is especialy true in systems employed in airplanes or vehicles Where the ambient noise level is high.

, In headphone receivers used under high noise level conditions, much of the external noise may be transmitted from three sources into the air cavity formed by the earcap. The noise may be transmitted through the earcap, between the earcap and the head of the listener, or through the bone and flesh structure of the listener.

It is an object of this invention to provide an improved acoustic device for use with earphones for reducing noise reaching the ear of a listener.

' It is a further object of this invention to provide an improved noise reduction system for use with earphones in which the tendency to oscillate is greatly reduced.

Other objects and advantages of the present invention all become apparent and immediately suggest themselves to those skilled in the art to which the invention is directed from a reading of the following specification in connection with the accompanying drawing.

In accordance with the invention, a noise reduction system includes an earphone having a radiating surface which radiates into an air cavity. A microphone is disposed in close proximity to the earphone and translates the ambient noise into electrical signals. The electrical signals are applied to a phase shifting and attenuation network. The electrical signals from this network are then applied to the earphone and are translated into acoustical signals of substantially equal amplitude and 180 degrees out of phase with the noise passing into the air cavity directly adjacent to the radiating surface of the earphone. tends to neutralize and eliminate the noise component directly applied to the air cavity from outside the earphone.

In the drawing:

Figure 1 is a diagram, partly in schematic form, of a noise reduction system embodying the present invention; and

Figure 2 is an enlarged view, partly in section, of the microphone and earphone shown in Figure 1.

Referring to the drawing,'a noise pickup microphonephone is wornby the operator. Such an earcap may be directly attached to the casing 11. The earcap and radiating surface forms an air cavity 13 between the earcap and a wearer of the earphone.

The reproduced noise component, therefore,

Patented Feb. 14, 1961 Figure 2 illustrates the construction of a typical telephone receiver that may be embodied into the present invention. It has two coils of wire 14 and 15 wound in many layers around two iron core pieces 16 and 17 to produce two electromagnets. A vibratile element or metal diaphragm 18 is so supported that it may be attracted by the magnetic poles or released depending upon the strength of magnetism in the poles. An increase of current through the windings causes the diaphragm to be attracted and a decrease of current causes it to be released, since the strength of the magnetic attraction is likewise increased or decreased. I

The radiating surface refers to the surface from which the acoustic signals enter into the air cavity 13. Most of these acoustic signals will be from the aperture 20 in the earcap 12. radiating surface may be much difierent than the sound produced directly by the diaphragm 18.

The microphone 10 may be of the conventional type which is responsive to the pressure of sound waves impinging upon it.

Ambient noise striking the microphone is converted into electrical energy which developes a voltage across a resistor 21 after passage through a pair of conductors 22 and 23. Conductor 22 is connected to a point of reference potential designated as ground. The voltage across the resistor 21 is applied across the input circuit of a preamplifier tube 24, Whichcircuit is connected to the grid 25 and the cathode 26 of the tube 24. A

resistor 27 and a capacitor 28 provide means for self- A plate load. resistor 29 is connected to a source of operating poten-.

biasing the tube in a well-known manner.

The function of this network is to correct for various.

irregularities in the frequency response and phase changes caused by the various acoustical and electrical compo? nents connected with the noise reduction system. For

example, such a network may comprise various resistor, capacitor and inductor arrangements. This network is: designed to respond in a predetermined manner overv a relatively wide band of audio frequencies.

The output from the phase shifting and attenuation network is applied across the input circuit of a second amplifier tube 33, through a conductor 34, connected.

'to the grid 35. A capacitor 36 and a resistor 37 are connected between the cathode 38 and ground to provide' means for self biasing the tube 33 during operation. A

load resistor 39 is connected to the source of operating;

potential.

The output from the second amplifier is coupled from the plate 40 to the grid 41 of a final amplifier tube 42. through a capacitor 43. A resistor 44 provides the grid leak for the tube. A resistor 45 and a capacitor 46 are connected between the cathode 47 and ground to pro-:

vide means for self biasing the tube during operation.

The output voltage from the final amplifier is applied from the plate 48 across the primary winding 49 of a transformer.

51 and 52. The screen grid 53 of the final amplifier is connected to the source of B+ potential.

noise reaching the air ca'vi'tyformed by the earcap from the three sources previously mentioned. v.

It may, therefore, be seen that the sound component produced by the noise entering the cavity of the ear It is seen that the sound from the The secondary winding '50 is connected; across the coils 14 and 15 through a pair of conductors.-

Due to the proper phasing and attenuation of the original voltage caused by the ambient noise striking the microphone, the voltage applied to theearphone produces asound 1 8,0; degrees out of phase and equal .in magnitude to the cap, when, worn by the operator will combine with the sound component produced by the movement of the vibratile element or diaphragm in response to the voltage applied across the secondary windingj41. In this way, the two sound components tendto eliminate one another, thus lowering therioisev level inside the earcap.

It" should be noted thatthe phase shift and attenuation network must bedesignedfor a specific set of components since,'for example, two microphones of different design will not have the, same phase shift and frequency characteristics.

A pair of input terminals 63 and 64 is provided to permit the application of audio signal through a resistor 65 to the input circuit of: the final amplifier tube 34. This permits the final amplifier totserve a dual purpose,

' namely, providing anoise reduction signal and, at the same time, producing an audio signal for'communication. The proper design of the system.will prevent the two separate functions of the amplifier from. interfering with one another.

Since the microphone is located outside the air cavity, the earcap provides some sound insulation between the air cavity and the pick-up microphone. This arrangement greatly aids in preventing acoustic feedback within the system.

It is seen from the foregoing description that a noise reduction system has been provided which is relatively simple and inexpensive. Modifications of the embodiment shown are, of course, possible without departing from the scope of the: invention.

' What is claimed is:

1. In a communication system including an earphone having a transducer for; actuating a vibratile element to generate acoustic waves in response to electrical operating signals, a noise reduction system comprising another transducer located adjacent said earphone for translating acoustic waves into electrical signals, said other transducer being disposed in the path of noise waves from the ambient directed upon said earphone whereby said noise waves are translated by said other transducer into corresponding electrical noise signals, a network having a predetermined phase shift and attenuation characteristic, means for applying said noise signals from said other transducer to said network, means for applying said noise signals from the output of said network to said firstnamed transducer for operating said transducer to generate an acoustic wave corresponding to said noise waves and substantially 180 degrees out of phase therewith, and means for applying other electrical signals representing an audio input to be translated into audio signals to said first named transducer simultaneously with said noise signals from the output of said network.

' 2. In a communications system including an earphone positionable adjacent the' head of a wearer and having a vibratile element for translating electrical signals into acoustical signals, a noise reduction system comprising a first transducer located in close proximity to said earphone and exposed with said earphone to ambient acoustic waves for translating said acoustic waves into electrical signals, means electrically coupled to said first transducer including a phase shifting and attenuation net-' work to provide said translated electrical signals with a; predetermined phase and amplitude, means for coupling the output of said network to said earphone for vibrating said element in response to said translated electrical signals for converting said electrical signals into acoustic signals substantially equal in amplitude and substantially 180 degrees out of phase with said ambient acoustic waves, and means for applying audio frequency signals to be heard by the wearer to said earphone simultaneously with said electrical signals.

3. In' a communications system including-an earphone adapted tobe worn on the head of a person with whom communication is intended, a noise reduction system comprising a microphone loeated'in close proximity to said earphone and exposed with said earphone to undesired ambient acoustic waves for translating said acoustic waves into electrical signals, means including a phase shifting and attenuation network to provide said translated electrical signals with a predetermined phase and amplitude, said network having an input and an output circuit, means for applying said translated electric-a1 signals from said microphone to said. input circuit, amplifier 5 a source of audio signals to be communicated through said earphone, and means for coupling said source to said amplifier means.

4. An acoustic system to reduce the acoustic noise reaching the ears of a person wearing an earphone, said earphone having. a sound radiating surface disposable opposite the ear of the wearer, said system comprising a noise pickup microphone disposed outside of said earphone in close proximity thereto, said microphone having means operable to convert the noise into corresponding electrical signals, a phase shifting and attenuation network coupled to said microphone to provide'saidcorresponding electrical signals with a predetermined phase and amplitude, means for applying said electrical signals from said network to said earphone to produce-an acoustic signal equal in magnitude and degrees out of phase with the noise at the radiating surface of said earphone, and means for applying audio signals to said earphone together with said signals from said network.

5. An acoustic system to reduce the acoustic noise reaching the ears of a person wearing an earphone having a transducer provided with a sound radiating surface, said system comprising a noise signal pickup microphone disposed outside of said earphonein close proximity thereto, said microphone having means operable to convert the noise into corresponding electrical signals, a phase shifting and attenuation network to provide said corresponding electrical signals with a predetermined phase and amplitude, means for coupling said micro phone to said network, means for applying said electrical signals from said networkto said transducer to produce an acoustic wave at said radiating surface equal in magnitude and 180 degrees out of phase with the noise at said transducer, and means for applying audio frequency signals to be translated into audible waves to said trans-- ducer simultaneously with said signals from said network.

6. In a communications system, a noise reduction system operative over a relatively wide band of audio frequencies including an earphone having a transducer for translating an electrical input into acoustic signals, said noise reduction system comprising an earcap in said earphone disposable around the ear of a wearer and cooperable with the head of the wearer to define an air cavitytherebetween, a microphone for translating ambient noise into electrical signals, said microphone being disposed in close proximity to said earphone, a phase shifting and attenuation network to, provide said electrical signals with a predetermined phase and amplitude, said network having an input and an output circuit, means for system operable over a relatively wide band of audio frequencies including an earphone for translating an electrical input into acoustic signals and having an earcap.

to form an air cavity between'said earphone and the head of a wearer, said noise reduction system comprising a microphone for translating ambient noise into electrical signals, said microphone being disposed in close proximity to said earphone, said earcap being disposed in the path of said noise between said microphone and the head of the wearer, an amplifier for said electrical signals, means connecting said microphone to the input of said amplifier, a phase shifting and attenuation network to provide said electrical signals with a predetermined phase and amplitude, said network having an input circuit and an output circuit, means for applying said electrical signals from the output of said amplifier to said input circuit, a second amplifier for said electrical signals, means connecting said output circuit to the input of said second amplifier, a final amplifier, means connecting the output of said second amplifier to said final amplier, means for applying other electrical signals representing an audio input to said final amplifier, and means for connecting said final amplifier to said earphone whereby said electrical signals are translated into acoustic signals substantially equal in amplitude and substantially 180 degrees out of phase with the noise entering said air cavity and said other electrical signals representing an audio input are translated into acoustic signals for communication purposes.

8. In a communication system including an earphone having a transducer for actuating a vibratile element to generate acoustic waves in response to electrical operating signals, a noise reduction system comprising another transducer for translating acoustic waves into electrical signals, said other transducer being disposed in the path of noise waves from the ambient directed upon said earphone whereby said noise waves are translated by said other transducer into corresponding electrical noise signals, a network having a predetermined phase shift and attenuation characteristic, means for applying said noise signals from said other transducer to said network, means for applying said noise signals from the output of said network to said first named transducer for operating said transducer to generate an acoustic wave corresponding to said noise waves and substantially degrees out of phase therewith, and means for applying other electrical signals representing an audio input to be translated into audio signals to said first named transducer simultaneously with said noise signals from the output of said network.

References Cited in the file of this patent UNITED STATES PATENTS 2,043,416 Lueg June 9, 1936 2,420,933 Crawford May 20, 1947 2,462,532 Morris Feb. 22, 1949 2,501,327 Good Mar. 21, 1950 2,616,971 Kannenberg Nov. 4, 1952 FOREIGN PATENTS 762,121 France Apr. 4, 1939 

